Auditor general Michael Pickup said in a report released Wednesday that the board serving Digby, Yarmouth and Shelburne counties is not addressing poor student achievement, especially in math and reading.
The 6,100 students under the board's jurisdiction have some of the poorest literacy and numeracy results in the province, he said.
Pickup's report says the board has not ensured that school officials are held accountable for the delivery of improved performance by students.
"Roles and responsibilities need to be clearly defined so that they are understood by both the governing board and management," it says.
Casey said she will choose a ministerial adviser to work with the board to improve oversight.
Improvements are expected over the next six months and the adviser will be required to provide her with monthly progress reports, Casey said.
She said the auditor's report has left her with one question: "Is there something that is not happening at the board level with respect to their policy ... which is translating into results that we're not happy with."
The Tri-County board said it would work to address the problems found by the auditor general.
Board chair Donna Tidd also welcomed Casey's directive, which was included in a letter sent later on Wednesday.
"Having someone come in from the outside to help us is a good thing and we welcome that help," said Tidd.